In existing semiconductor devices, transistors communicate with one another via an elaborate series of copper interconnects connected through a series of metal layers above the transistor. To minimize the capacitive coupling between these interconnects, the space between is occupied by a material with a low dielectric constant (i.e., low-K materials). To prevent the diffusion of copper into this low-K material, a composite barrier is put in place. Current practices use physical vapor deposition techniques to accomplish this. An example of a BEOL (back end of the line) interconnects strategy for putting a barrier in place by physical vapor deposition and electrochemical deposition is as follows: low K repair, tantalum nitride reactive sputter physical vapor deposition, tantalum sputter physical vapor deposition, copper seed sputter physical vapor deposition and copper electrochemical deposition.
Physical vapor deposition techniques result in anisotropic deposition, with the thickness of the film on sidewalls being significantly thinner than the thickness of the film on the horizontal surfaces of the wafer. Since the ability of the barrier to prevent the migration of copper through to the low-K dielectric is proportional to the thickness of the barrier, the barrier is thicker than it needs to be on the horizontal wafer surfaces.
As the semiconductor moves to future technology nodes, the dimensions of interconnects will decrease. This will result in a decrease of the surface area to volume ratio of the interconnect, concomitant with an increase in the volume occupied by the diffusion barrier. As the barrier occupies more of the interconnect channel space, the effective resistivity of the interconnect increases for two reasons: first, decrease in the size of the interconnect and second, copper/barrier surface scattering of electrons becomes a more critical issue.
One method of minimizing these issues is to deposit films isotropically using atomic layer deposition. Unfortunately, no chemistries exist that can deposit tantalum metal using atomic layer deposition. The role of tantalum in the deposition strategy described above is to generate adequate adhesion between the copper seed and tantalum nitride. Without tantalum, copper delaminates from the tantalum nitride film compromising device performance.
Another metal that may be viable within this application is ruthenium. Ruthenium is adherent to titanium nitride and thus one may expect that it would be adherent to tantalum nitride, moreover, the use of ruthenium could obviate the requirement of a copper seed layer since ruthenium has sufficient conductivity that copper electrochemical deposition could be carried out directly on a ruthenium film. An isotropic atomic layer deposition strategy for forming BEOL interconnects using ruthenium is as follows: low K repair, tantalum nitride atomic layer deposition, ruthenium atomic layer deposition and copper electrochemical deposition.
While there have been reports in the literature detailing ruthenium atomic layer deposition, all of them involve the use of either oxygen or a plasma. Oxygen based chemistries are incompatible with a BEOL integration sequence since the presence of trace amounts of oxygen within the deposited film could diffuse into the copper channel resulting in the formation of copper oxides compromising device performance. Similarily, concerns exist regarding the ability of plasmas to deposit isotropic films.
Ideally, a suitable BEOL atomic layer deposition process would be capable of using hydrogen, or other reducing gas, at temperatures below 300° C. so that the deposition could be carried out in a manner compatible with the rest of the BEOL integration strategy. In addition to being hydrogen reducible, chemistries should deposit in a self-limiting manner. In other words, in the absence of a reactant gas, the substrate should saturate with a monolayer, or fraction of a monolayer, of a dissociatively chemisorbed precursor.
The problem is that there are no known suitable hydrogen reducible ruthenium complexes of sufficient volatility for use as atomic layer deposition precursors, and as such, no self-limiting, hydrogen reducible precursors have been identified. It would therefore be desirable in the art to develop self-limiting, hydrogen reducible ruthenium complexes suitable for BEOL atomic layer deposition processes.
Further, the synthetic processes utilized to generate organometallic precursors are highly important, and must insure safety, high purity, throughput, and consistency. The economics associated with such processes together with the rigid requirements of the electronics industry make the synthesis of organometallic precursors challenging. Developing a methodology for producing organometallic precursors that addresses the aforementioned potential hold-ups would be beneficial toward establishing the production of these materials for use in the electronics industry.
Processes for preparing organometallic compounds include those disclosed in U.S. Patent Application Publication No. US 2004/0127732 A1, published Jul. 1, 2004. Organometallic precursor compounds may also be prepared by processes such as described in Vendemiati, Beatrice et al., Paramagnetic Bis(amidinate)Iron(II) Complexes and their Diamagnetic Dicarbonyl Derivatives, Euro. J. Inorg. Chem. 2001, 707-711; Lim, Booyong S. et al., Synthesis and Characterization of Volatile, Thermally Stable, Reactive Transition Metal Amidinates, Inorg. Chem., 2003, Preprint; and references therein.
A need exists for new processes for making organometallic precursors that give higher product yields, operate efficiently, provide consistency and permit easier scale up for production quantities of organometallic compounds. It would therefore be desirable in the art to provide new processes for making organometallic compounds that address these needs.
Also, in developing methods for forming thin films by chemical vapor deposition or atomic layer deposition methods, precursors that preferably are hydrogen reducible, deposit in a self-limiting manner, liquid at room temperature, have adequate vapor pressure, have appropriate thermal stability (i.e., for chemical vapor deposition will decompose on the heated substrate but not during delivery, and for atomic layer deposition will not decompose thermally but will react when exposed to co-reactant), can form uniform films, and will leave behind very little, if any, undesired impurities (e.g., halides, carbon, etc.) are highly desirable. A need exists for developing new compounds and for exploring their potential as chemical vapor or atomic layer deposition precursors for film depositions, in particular self-limiting, hydrogen reducible organometallic complexes for atomic layer deposition as indicated above. It would therefore be desirable in the art to provide precursors that possess some, or preferably all, of the above characteristics.